Generally, in variable speed motor systems adopting an inductor as a power generation motor, an accidental over-voltage or over-current may be induced to a secondary-winding side of a power generator due to a problem on the electric power system, and a power converter which supplies an excitation current having a variable frequency may be damaged as a result of the induced over-current. Over-current and over-voltage prevention methods applying a short-circuit device are generally used to prevent the above problems.
The over-current and over-voltage prevention methods applying a short-circuit device of the variable speed motor systems generally utilize an over-voltage prevention device comprising a short-circuit device connected to a three-phase electric device to be protected from an over-voltage in which thyristor valves, each having thyristors connected in series, are connected by a delta connection, a flip-flop to be set by an over-voltage being applied to the three-phase electric device, an AND circuit to provide a trigger signal to the thyristor valves under an AND condition of a forward voltage signal of the thyristor valves and an output of the flip-flop, a current detector which detects a current of each thyristor valve, and a means for resetting the flip-flop under a condition where an output of the current detector is zero for a predetermined period.
In the aforementioned over-current and over-voltage prevention methods of the variable speed motor systems, when a problem occurs on the electric power system, a transitional over-voltage or over-current occurs for either grounding or inter-short-circuiting in the variable speed motor systems. In this case, an over-voltage or over-current flows into the system in comparison with the normal operation, and an over-voltage or over-current is induced to a secondary-winding side of a wound-rotor induction machine. If the secondary-winding side of the induction machine does not have a short circuit device, an over-voltage or over-current directly flows into the power converter. To avoid this, it is necessary to provide a large-capacity converter in the case of a fault in the electric power system, which rarely occurs.
One of the solutions to this problem is to adopt a power generator system in which a secondary-winding side of a power generator is short-circuited by a first short-circuit device 7 having a small impedance, as shown in FIG. The first short-circuit device 7 has a circuit structure as shown in FIG. 6. With the first short-circuit device 7, in a band where a slip frequency of a motor is large, over-current flowing into the over-voltage prevention device significantly increases, and accordingly, a power converter 10, the capacity of which is determined based on a voltage, and a current in normal operation cannot open the over-voltage prevention device, and the secondary-winding side of the power generator cannot recover from the short-circuited status, thereby failing to ensure continuous operation.
Another solution is to adopt a power generator system in which a secondary-winding side of a power generator is resistance short-circuited by a second short-circuit device 8 having a minute impedance greater than the impedance of the first short-circuit device 7, as shown in FIG. 5. The second short-circuit device 8 has a circuit structure as shown in FIG. 7. If the resistance of the second short-circuit device 8 is sufficiently small, resistance R2 from windings of a rotor of the power generator to the short-circuit device 8 becomes sufficiently smaller than resistance R1 from a point of failure caused by a fault in the electric power system to windings of a stator of the power generator. Accordingly, a transitional DC current flowing into the second short-circuit device 8 of the secondary-winding side of the power generator, i.e., the decay rate of a transitional AC current generated at the primary side of the power generator, becomes slower than the decay rate of the transitional DC current flowing through the primary side of the power generator. In this case, the current becomes zero in a breaker 11, and the breaker 11 can be opened. However, if the resistance of the second short-circuit device is large, and resistance R2 becomes greater than R1, the aforementioned relation of decay rates will be inversed. In this case, the current of the primary side does not become zero, and the breaker 11 cannot block the current.
Under the above circumstances, it is desirable to provide an over-voltage prevention device, which is capable of recovering from a failure in the electric power system within a short time and ensuring continuous operation with a simple structure.